Check out this suggestion from Jack Canavera. I watched him take it apart -- small fingers and small combination/open ended wrenches will help. The soot up the sides was severe until the spider web was removed.

It's not a good situation. Here's my sage advice (worth about as much as a wooden nickel):

1. It's not the thermocouple. Thermocouples are safety devices that shut down the gas (propane, in your case) when they're cold. If the thermocouple is bad, then the pilot will light while you've got the button depressed, but it won't continue as flame after the button is released. (We just replaced two of them, one for a wall heater in the cabin, and the other for the water heater for the bathhouse.)

2. It probably is air mixture. You should first check for some kind of blockage. (A dead moth can really muck up the works!) If there's no blockage, then there is usually some kind of small plate near the propane entry line that allows you to adjust the amount of air coming in. Adjust to a nice, blue flame that doesn't hiss too much.